UpLink (UL) control channels are utilized to provide the base station with Hybrid Automatic Repeat reQuest (HARQ) feedback in response to DownLink (DL) data transmissions, Scheduling Request (SR) for the uplink as well as Channel State Information (CSI) reports corresponding to downlink carriers. For carrier aggregation, the Uplink Control Information (UCI) overhead can become extensive, considering the User Equipment (UE) can be scheduled on multiple downlink carriers simultaneously, which not only increases the HARQ feedback but also the need for more CSI reports to assist the scheduling.
The UCI overhead is even more severe for Time Division Duplex (TDD) systems, since there are typically less uplink subframes available than downlink subframes, which implies that the UCI payload becomes larger per transmission attempt of the uplink control channel. In Long Term Evolution Advanced (LTE-Advanced) Rel-12, a UE can be configured to aggregate up to 5 serving cells (i.e., different carriers in an eNodeB), where a serving cell comprises at least a downlink component carrier and where each carrier has at most 20 MHz bandwidth. The UE is always configured with a UE-specific primary serving cell having both a downlink and uplink component carrier. The uplink control channel is transmitted on the primary serving cell. Additionally, a UE can be configured to aggregate up to 4 UE-specific secondary serving cells.
The Physical Uplink Control Channel (PUCCH) in LTE-Advanced comprises several PUCCH formats, e.g., formats 1/1a/1b/2/2a/2b/3, each being used for a particular purpose of SR transmission, HARQ feedback and periodic CSI reporting. The payload of these PUCCH formats range from 1 to 22 information bits. PUCCH formats 1/1a/1b/2/2a/2b are based on modulated sequences, while PUCCH format 3 is using Discrete Fourier Transform (DFT) spread transmission. Several of the PUCCH formats also include Code Division Multiplexing (CDM) transmission, such that multiple UEs could be transmitting the PUCCH on the same Physical Resource Block (PRB) pair by utilizing different sequences. PUCCH formats 1/1a/1b/2/2a/2b could share the same PRB pair, while PUCCH format 3 PRB pairs cannot be shared with other PUCCH formats. A characterizing feature is that all the PUCCH formats utilize QPSK modulation and occupy only 1 PUCCH region, which comprises one PRB pair.
The resource allocation includes providing the UE with information such that it can derive what sequences and PRB pairs it should use for the PUCCH transmission. The PUCCH resources for PUCCH formats 2/2a/2b are semi-statically configured and would typically be located at the outer PUCCH regions close to the edge of the carrier. These formats are primarily used for periodic CSI reporting. The PUCCH resources for PUCCH formats 1a/1b can be derived implicitly from the PDCCH or the Enhanced Physical Downlink Control Channel (EPDCCH) that was used for scheduling the associated Physical Downlink Shared Channel (PDSCH). Thus, they are allocated dynamically on a need basis. PUCCH resources for PUCCH formats 1a/1b could also be semi-statically configured for the case where there is no associated PDCCH/EPDCCH, e.g., for Semi-Persistent Scheduling (SPS). Typically, up to 18 UEs could be multiplexed within a PBR pair, utilizing different cyclic shifts of the transmitted sequence to achieve CDM transmission. PUCCH format 3 can, e.g., be used if the UE is scheduled on at least one secondary serving cell. Up to 5 UEs can be multiplexed within a PRB pairs by utilizing different spreading codes. A set of semi-statically PUCCH resources is configured for PUCCH format 3 and a resource is dynamically indicated from this set by an indicator in the associated PDCCH/EPDCCH on a secondary cell. If the UE is only scheduled on its primary serving cell, it is using PUCCH formats 1/1a/1b resources.